The borad aim of the work is to understand the regulation of DNA synthesis and cell multiplication in animal cells. The specific goals are, first, to purify and, if possible, to characterize the gorwth factors that, in addition to insulin, are required to chick embryo fibroblasts and baby hamster kidney cells for progression from a resing state into the period of DNA replication. The second group of aims includes finding changes in the functional levels of specific mRNAs during the Gl period, defining the growth factors that are responsible for the changes, and elucidating the mechanisms by which the factors induce the alterations. The last goal is to develop an efficient procedure for microinjecting mRNAs into cultured cells that is innocuous to the recipient cells. The procedure will be used to learn whether traversal of the Gl period is dependent upon the synthesis of mRNAs and will provide and assay system for studying the critical messages. The methods to be used are those of te biochemically-oriented cell biologist and include gel filtration, cell-free translation of mRNAs from resting and stimulated cells, cDNA synthesis and hybrid-arrested translation, two-dimensional gell electrophoresis in agarose gels, and encapsulation of messages in liposomes and erthyrocyte ghosts.